1. Field of the Invention
The present invention relates to devices that utilize evaporative, convective, and/or conductive cooling to cool the human body in aid of surgery, medical treatment, therapy, or comfort. Some exemplary cooling structures include various configurations of thermal cooling devices.
2. Description of the Related Art
Temperature control in humans has important medical consequences. In order to maintain optimum health, the human body must maintain a core temperature within a very narrow range. Core body temperature changes as small as 0.1xc2x0 Celsius trigger thermoregulatory responses such as vasoconstriction, vasodilation, shivering, or sweating. A narrow temperature range is optimal for human cellular functions, biochemical reactions, and enzymatic reactions. Outside this range of temperatures, the human body experiences hypothermia (excessive cold) or hyperthermia (excessive hot).
Hyperthermia can result from illness or environmental heat stress, among other causes. In some cases, healthy people suffer hyperthermia when their natural cooling mechanisms, such as sweating, are overwhelmed during heavy physical work in a hot environment. This situation can become even worse if the person fails to drink enough fluids, and therefore cannot sweat adequately. Heat stress disorders, categorized in ascending order of severity, include: heat cramps, heat syncope, heat exhaustion, and heat stroke. Normally, discomfort causes people choose to stop working before the onset of heat exhaustion, but competitive athletics or military activities sometimes push people beyond the limits of health.
Hyperthermia can also result from fever associated with illness. Fever may arise from infection, tumor necrosis, thyroid storm, malignant hyperthermia, brain injury, and other causes. Brain injuries that cause hyperthermia usually involve the hypothalamus, and may be caused by tumors, stroke, head injury, or cardiac arrest (in the case of ischemic brain injury).
Some consequences of hyperthermia include fluid and electrolyte imbalances, increased cellular metabolic rates, and cognitive impairment. More serious consequences include motor skill impairment, loss of consciousness, and seizures. Ultimately, hyperthermia can cause irreversible cellular injury (especially of the highly metabolic brain and liver cells), organ failure, and death. Hyperthermia is a condition that, depending on its severity, may require immediate cooling treatment to return the person""s core temperature to normal.
Cooling treatment may also have other important uses. In some situations, mild or moderate hypothermia is believed to provide beneficial protection against injury. Moreover, induced hypothermia can be beneficial when the blood flow to some or all of the brain has been interrupted. Brain ischemia due to an interruption of blood flow may occur during cardiac arrest, surgery on the blood vessels of the brain, stroke, traumatic brain injury, or open heart surgery. Cooling the brain before (or in some cases after) these events can protect the brain from injury, or at least decrease the severity of the ultimate brain damage.
Physicians have used various devices and techniques to cool the human body, including pharmacological cooling and various types of mechanically induced cooling. Mechanically induced cooling approaches generally fall into one of these categories: conductive, convective, or evaporative. While different implementations have been tried, many are limited by lack of practicality, difficulty of use, ineffectiveness, and/or excessive power consumption.
One example of conductive cooling involves packing a hyperthermic person""s body in ice, or immersing the person in cool or cold water. While ice is an effective cooling agent, it is painful to the person, potentially damaging to the skin, difficult to obtain in large quantities, and impractical for long term use. Water baths can be effective, although they are not practical for the comatose or intensive care patient, or for long term use. In one less effective, but common method of conductive cooling, a person may be placed in contact with a cold-water-circulating mattress and/or cover. Water inside the mattress removes heat from the person by conduction wherever the surface of the mattress thermally contacts the person""s skin. Although there is some benefit to such devices, they are often uncomfortable and heavy, and provide inefficient thermal contact because they are not precisely shaped to the body.
In contrast to conductive cooling, convective cooling involves blowing air onto a person. Convective cooling is the least effective method of cooling from a thermodynamic point of view. Room temperature air can be blown very inexpensively with a fan. However, its cooling effectiveness is severely limited due to the thermal capacity of air, and related heat transfer coefficients.
For more efficient convective cooling, the air can be cooled before being blown onto the person. Air can be cooled, for example, with a traditional compression or heat-pump air conditioner, vortex cooling, or with thermoelectric cooling. Cooled air can also be generated using the xe2x80x9cswamp coolerxe2x80x9d principle of vaporizing water into the air stream. When water evaporates into the air, it cools the air. Then, the cooled air is applied to a person.
After the air is cooled with one of the foregoing techniques, it can be delivered to a person by cooling the air in the person""s room. To save energy, cooling can be confined to the person rather than the whole environment surrounding the person. One technique that uses this approach is the convective thermal device, which has been implemented in a variety of forms.
Although convective cooling removes the stress of environmental heat, it is minimally effective in active cooling. This limited thermodynamic effectiveness is particularly evident when trying to cool people with fevers. Generally, in order to be cooled by convection, a feverish person must be anesthetized and paralyzed to prevent the body""s heat-producing shivering response. Further, due to the thermodynamic inefficiency of convective cooling, this method of cooling uses considerable electrical power and generates considerable waste heat, which can be a problem in emergency rooms or intensive care units.
Having discussed conductive and convective cooling, the final mechanically induced cooling mechanism is evaporative cooling. Sweating is a principal example of evaporative cooling. Because water has a large heat of vaporization, large amounts of heat can be removed from the body by evaporating relatively small amounts of water. For example, when a gram of water evaporates, it extracts 540 calories of heat (2.26 kJ) from the skin. On hot summer days, many people practice basic evaporative cooling by wetting their skin or clothing with water, and permitting the water evaporate. Medical staff employ evaporative cooling by giving sponge baths to patients, where the unclothed patient is wetted with water and allowed to dry by evaporation. Sometimes a fan is pointed at the person to increase the evaporation rate. While sponge baths are indeed effective, they are labor intensive, messy, demeaning to body-conscious people, and impractical for prolonged cooling. Finally, evaporative cooing has limited effectiveness in high humidity environments.
Therefore, as shown above, conductive, convective, and evaporative cooling systems each have certain benefits and limitations. And, although some of the foregoing cooling products have certain advantages and might even enjoy some commercial success, engineers at Augustine Medical, Inc. are continually seeking to improve the performance and efficiency of human cooling systems. Some areas of possible focus include simplifying hardware designs, boosting the effectiveness of cooling systems, and cooling specific body parts.
An additional area of focus concerns the management of the liquid source during evaporative cooling. Introducing too much liquid causes liquid to spill over the area of focused cooling (the xe2x80x9ccooling fieldxe2x80x9d), and pool under the person. Pooling of contaminated liquids presents hygienic and esthetic problems in the medical environment. On the other hand, if too little liquid is supplied, the cooling field may dry out and stop or reduce cooling effectiveness.
Broadly, the present invention introduces cooling devices that utilize evaporative, convective, and/or conductive cooling to reduce a person""s temperature in aid of surgery, medical treatment, therapy, or comfort.
One improved cooling device maximizes evaporative cooling by aiding the flow of air to the body and also removing vapor-laden air from the body. This device includes an upper sheet and a base sheet adhered in many locations such that the sheets cooperatively form an inflatable structure. The inflatable structure includes numerous elongated, substantially parallel, inflatable cooling chambers, and between each pair of neighboring cooling chambers, a connecting membrane. The inflatable structure also includes numerous ventilating cross-members, each of which spans and interconnects the cooling chambers. Air enters the chambers through an air inlet in the inflatable structure, and then exits the chambers toward the person through numerous exhaust holes located in the base sheet. After being heated by the person""s body, this air can exit the inflatable structure through evaporation openings in the connecting membranes.
In a different embodiment, upper and base sheets may be adhered in different locations to provide a cooling device with a cooling chamber that forms a continuous a serpentine path. The serpentine cooling chamber is held in this configuration by connecting membranes between neighboring segments of the path. In this embodiment, ventilating cross-members are unnecessary because all regions of the cooling chamber are in fluid communication with each other. After air enters the chamber through an air inlet in the structure, the air exits the chamber toward the person through numerous exhaust holes located in the base sheet. After being heated by the person""s body, this air can exit the inflatable structure through evaporation openings defined in the connecting membranes.
Another embodiment of the invention concerns a generally rectangular cooling device that includes certain body-conforming features. Namely, the device has a number of body-contour slits extending inward from the perimeter. Due to the slits"" locations, they permit the inflatable structure to conform to a person""s legs and outstretched arms.
Still another embodiment of the invention concerns an inflatable cooling device that includes an evaporative cooling layer. This layer comprises a sheet of absorbent material capable of holding a substantial amount of water. The absorbent sheet is placed in thermal contact with the person""s skin, saturated with water, and then evaporatively cooled by the overlying thermal cooling device. As this layer cools (by evaporation), it has the effect of cooling the person (by conduction). The absorbent sheet may advantageously comprise a super-absorbent material such as starch-grafted sodium polyacrylate.
Accordingly, as discussed above, the invention may be implemented to provide various types of apparatus, such as cooling devices described herein. In contrast, certain other embodiments concern methods for utilizing such apparatuses.
The invention affords its users with a number of distinct advantages. By using a blower to induce evaporative cooling, the invention avoids the need for power consuming refrigeration equipment. As another advantage, evaporative cooling is thermally self-limiting, because it will not produce surface temperatures that would freeze skin, as is the case with ice packs. Also, unlike ice and other phase change materials that can only maintain fixed temperatures for a limited time, cooling with this invention can be sustained indefinitely by periodically adding water to the cooling field.
As another advantage, absent from previous approaches, this invention may incorporate a super-absorbent material into cooling devices. The super-absorbent material is capable of holding a large volume of water relative to its mass. This material, once wetted, can provide hours of evaporative cooling without the need for a liquid reservoir and piping system to replenish the cooling field. Consequently, cooling devices of this invention help discourage the introduction of too much cooling liquid, and help prevent the cooling liquid from overwhelming the cooling field and spilling over.
Furthermore, the cooling devices of this invention include improved means to direct air flow through the device, maximizing delivery of dry air to all areas of the cooling field. Additionally, the invention employs ventilation openings to aid the escape of saturated vapor from the cooling field and allow an increased influx of dry air. The invention also provides a number of other advantages and benefits, which should be apparent from the following description of the invention.